1. Field of the Invention
The present invention relates to an X-ray computer tomographic imaging apparatus and control method thereof.
2. Description of the Related Art
Recent advances in medical diagnostic apparatuses are remarkable. Especially in the field of X-ray computer tomographic imaging apparatuses (X-ray CT apparatuses) that get tomographic images of a object using X-rays, various efforts have been made constantly by, for example, diversifying their applications or shortening the photographing time.
In parallel with such progressive technical innovations, the problem of how to dissipate heat generated collaterally in the apparatus to the outside has always been considered. However, the more the performance or function of the apparatus is improved, the more the amount of heat to be dissipated increases, with the result that the problem hangs heavy.
Typical heat generated in the X-ray CT apparatus is attributable to a regenerative resistor. A regenerative resistor is a resistive member provided to convert into heat energy the energy of back electromotive force generated in decelerating the rotation of a motor (such as a direct drive motor or a rotation servo motor) for driving the rotating frame to get tomographic images of a object while rotating. Therefore, when the motor is accelerated and decelerated frequently, for example, when a large number of people are photographed consecutively, or when a service engineer does maintenance, the regenerative resistor may reach a considerable temperature (about 70 degrees). Known configurations to deal with such a situation include not only the one that simply has an increased number of regenerative resistors but also, for example, the ones explained below.
The gantry of some X-ray CT apparatuses have a rectangular appearance when viewed from the front. Such X-ray CT apparatuses often employ the following configuration: a regenerative resistor is provided in the upper part of the gantry in such a manner that it is arranged so that heat may easily dissipate outside the apparatus and is brought into contact with the sheet metal in the gantry to allow heat to escape from the sheet metal.
In recent years, an X-ray CT apparatus has been popularized which has the upper part of the gantry formed into a round shape to give the apparatus a soft image in order to wipe away the mechanical image of the apparatus, thereby relieving discomfort the object undergoes. In such an X-ray CT apparatus, since it is difficult to secure a sufficient space to arrange a regenerative resistor in the upper part, a regenerative resistor is often provided on the apparatus side.
The aforementioned X-ray CT apparatus uses as an additional configuration element a fan or the like to guide heat generated by a regenerative resistor to the outside. For example, Jpn. Pat. Appln. KOKAI Publication No. H9-276262 mentioned below has disclosed an X-ray CT apparatus which has a suction opening made in the upper part of the photographic opening and a cooling fan provided in the upper part of the apparatus to generate an airflow inside the apparatus, thereby dissipating heat.
In addition, Jpn. Pat. Appln. KOKAI Publication No. H9-56710 mentioned below has disclosed an X-ray CT apparatus (computerized traverse axial tomography) which is configured to arrange a plurality of blade members at a support member and rotate the blade members together with a gantry rotating unit to send air, thereby dissipating heat from inside the apparatus. Like the configuration of Jpn. Pat. Appln. KOKAI Publication No. H9-276262, the configuration of Jpn. Pat. Appln. KOKAI Publication No. H9-56710 is such that it dissipates heat by keeping good ventilation inside the apparatus.
Furthermore, Jpn. Pat. Appln. KOKAI Publication No. 2002-336236 described below has disclosed an X-ray CT scanning system which includes a regenerative resistor unit provided in a gantry apparatus (X-ray CT apparatus) and a blower fan that transfers the heat generated by the regenerative resistor unit to the top board of the carrier unit on which the object is laid. In the X-ray CT scanning system, heating the top board makes it possible to warm up the object. In this case, too, a method of cooling the regenerative resistor by airflow is used.
Here, as an example, a conventional X-ray CT apparatus which has a configuration that dissipates heat through a regenerative resistor will be explained below using drawings. FIG. 1 is a perspective front view schematically showing the configuration of a conventional X-ray CT apparatus. FIG. 2 is a block diagram showing the configuration of the conventional X-ray CT apparatus. As shown in FIG. 1, the X-ray CT apparatus 1 is an apparatus which irradiates X rays to a object, while scanning the object, and detects the transmitted X rays. The X-ray CT apparatus 1, a couch for transporting the object laid on the top board to a photographing position (or photographic opening shown below), and a computer for analyzing the detected data from the X-ray CT apparatus 1 and restructuring an X-ray tomogram (neither of which is shown) constitute a tomographic X-ray system.
An opening made near the center of the body 2 of the X-ray CT apparatus 1 forms a photographic opening 3 into which the object laid on the top board is to be inserted. The body 2 houses various devices for irradiating X rays to the object in various directions and detecting the X rays passed through the object. The devices include a motor 4, such as a direct drive motor, a rotating frame 5, and a servo amplifier 6. A regenerative resistor 7 is connected to the servo amplifier 6.
The rotating frame 5, which is a frame provided so as to enclose the photographic opening 3, is rotated by the motor 4. On the rotating frame 5 (support means), an X-ray tube 8 (X-ray generating means) which outputs X rays and a detector 9 (detecting means) which detects X rays output from the X-ray tube 8 are supported in opposed positions. Moreover, the rotating frame 5 is provided with a power unit 10 for supplying electric power to the X-ray tube 8 and the detector 9 and a signal processing unit 11 for processing the result of detection by the detector 9.
The servo amplifier 6 adjusts the voltage and frequency of the electric power supplied to the motor 4 on the basis of a signal transmitted from a control unit, thereby driving and stopping the motor 4 or controlling the rotating speed.
The regenerative resistor 7 is a member for converting electric energy (regenerative energy) generated during the deceleration of the motor and flowing backward into heat energy. A regenerative resistor is also provided in the servo amplifier 6. It is a regenerative resistor 7 that is used to consume the regenerative energy the built-in regenerative resistor cannot deal with. The regenerative resistor 7 is provided in the upper part of the side of the body 2 of the X-ray CT apparatus 1 and is thermally connected to a heat-dissipating member, thereby dissipating heat to the outside.
The individual members arranged as described above constitute a configuration as shown in FIG. 2. As shown in FIG. 2, the power unit 10 is connected to the motor 4 via the servo amplifier 6 composed of an IGBT (Insulated Gate Bipolar Transistor). The regenerative resistor 7 is inserted in the transmission path between the power unit 10 and the IGBT. On the regenerative resistor 7 side, there is provided a switch SW1 controlled by the control unit 100 to send the regenerative energy generated during the deceleration of the motor 4 to the regenerative resistor 7 connected to the heating-dissipating member.
In the X-ray CT apparatus configured as described above, a photographing process by the tomographic X-ray system including the X-ray CT apparatus 1 is basically executed in the following process. The X-ray CT apparatus I not only supplies electric power from the servo amplifier 6 to the motor 4 and rotates the rotating frame 5 but also irradiates X rays from the X-ray tube 8, thereby detecting with the detector 9 the X-rays passed through the object inserted in the photographic opening 3. At this time, the X-ray tube 8 and detector 9 are in operation, receiving the electric power supplied from the power unit 10. The transmitted X rays detected by the detector 9 are processed by the signal processing unit 11, which produces image data and transmits the image data to the computer. Then, the computer restructures the image data into an image, thereby providing a tomogram of the object.
As a result of the repetition of the aforementioned photographing process, as shown in FIG. 3, energy based on back electromotive force during the deceleration of the motor 4 (sensed by the control unit 100 in S11), that is, regenerative energy, is generated in large amounts. When the control unit 100 turns on the switch SW1 (S12), the regenerative energy is sent to the regenerative resistor 7, with the result that the heat-dissipating member thermally connected to the regenerative resistor dissipates heat.
As was mentioned at the beginning, X-ray CT apparatuses are continually making progress and particularly an attempt to reduce the burden on the object by shortening the photographing time is now in progress. To shorten the photographing time, it is necessary to shorten the scanning time. Therefore, it is necessary to rotate the rotating frame at higher speed, that is, to control the motor in high-speed rotation. To realize this, the motor has to be accelerated and decelerated rapidly, resulting in the generation of a large amount of regenerative energy as compared with a conventional equivalent. Thus, the technique for converging a great deal of regenerative energy generated into heat energy efficiently is required. That is, when the simplification of the configuration and manufacturing costs are taken into consideration, the so-called air-cooled heat-dissipating functions described in above undeniably have their limits in dealing with an increase in the dissipation of heat energy resulting from the speeding up of the rotation of the rotating frame. Consequently, it is anxious that various precision instruments will malfunction due to a temperature rise in the apparatus.